Some turbines must operate at extremely high temperatures and at very high angular velocities. For example, turbines of the type employed in intercontinental cruise missiles or tactical missiles may operate at temperatures up to 3500 degrees F. and have a blade tip speed or tangential velocity of about 2000 feet per second.
The high angular velocities of the turbine rotor produces high centrifugal force. The strength of many materials reduces drastically at temperatures at or below 3500 degrees F. Accordingly, there are very few known materials which can be used in the rotor of a turbine of this kind. In fact, material selection is essentially limited to a few ceramics and a composite material known as carbon/carbon. Carbon/carbon comprises a carbon matrix with carbon fibers oriented to obtain the desired directional strength characteristics. The carbon is coated in accordance with known practice to prevent the carbon from oxidizing at high temperatures.
A rotor for a turbine of this kind typically includes a rotatable hub, rotor blades mounted on the hub and a shroud rotatable with the rotor and lying radially outwardly of a portion of the blades. One problem with this construction is that the shroud tends to fail when the rotor is spun at high angular velocities.